United States Environmental

Protection Agency

Office of Water

Washington, DC 20460

EPA-822-R-02-025

March 2002

Methods for evaluating wetland condition

#17 Land-Use Characterization for

Nutrient and Sediment

Risk Assessment
United States Environmental

Protection Agency

Office of Water

Washington, DC 20460

EPA-822-R-02-025

March 2002

Methods for evaluating wetland condition

#17 Land-Use Characterization for

Nutrient and Sediment

Risk Assessment

Principal Contributor

Iowa State University

Arnold van der Valk

Prepared jointly by:

The U.S. Environmental Protection Agency

Health and Ecological Criteria Division (Office of Science and Technology)

and

Wetlands Division (Office of Wetlands, Oceans, and Watersheds)
United States Environmental

Protection Agency

Office of Water

Washington, DC 20460

EPA-822-R-02-025

March 2002

Notice

The material in this document has been subjected to U.S. Environmental Protection Agency (EPA)

technical review and has been approved for publication as an EPA document. The information

contained herein is offered to the reader as a review of the “state of the science” concerning wetland

bioassessment and nutrient enrichment and is not intended to be prescriptive guidance or firm advice.

Mention of trade names, products or services does not convey, and should not be interpreted as

conveying official EPA approval, endorsement, or recommendation.

Appropriate Citation

U.S. EPA. 2002. Methods for Evaluating Wetland Condition: Land-Use Characterization for

Nutrient and Sediment Risk Assessment. Office of Water, U.S. Environmental Protection Agency,

Washington, DC. EPA-822-R-02-025.

This entire document can be downloaded from the following U.S. EPA websites:

http://www.epa.gov/ost/standards

http://www.epa.gov/owow/wetlands/bawwg

ii
iii

Contents

Foreword ................................................................................................................... v

List of “Methods for Evaluating Wetland

Condition” Modules................................................................................................vi

Summary ...................................................................................................................... 1

Purpose....................................................................................................................... 1

Introduction.............................................................................................................. 1

Methods...................................................................................................................... 1

Nutrient Loading Index........................................................................................... 5

Case Study ................................................................................................................. 7

References ............................................................................................................. 10

List of Tables

Table 1: Potential Nutrient Loss Rates

(kg/ha/yr) for Different Upland

Land-Use Classes ............................................................................... 5

Table 2: Land Uses (ha) in the Watersheds of

Wetlands W1, W2, and W3 in the Iowa

Great Lakes Region of NW Iowa ..................................................... 7

Table 3: Estimated Current Annual Nitrogen and

Phosphorus Loadings (Total Watershed

Loss), Presettlement Loadings (Natural

Watershed Loss), and Nutrient Loading

Index for Three Wetlands (W1, W2, and

W3) in the Iowa Great Lakes Region ............................................. 8
iv

Table 4: Percent of the Watersheds for Wetlands

W1, W2, and W3 Classified as Agricultural

Land, Percent of Agricultural Land

Classified as Highly Erodible (HEL),

Percent of Wetland Boundary Adjacent

to Agricultural Land, and Percent of

Watershed Disturbed by Recent Land

Clearing for Three Watersheds in the

Iowa Great Lakes Region As Well As the

Sediment Risk Index for Each Wetland ....................................... 9
v

Foreword

In 1999, the U.S. Environmental Protection Agency (EPA) began work on this series of reports entitled

Methods for Evaluating Wetland Condition. The purpose of these reports is to help States and

Tribes develop methods to evaluate (1) the overall ecological condition of wetlands using biological

assessments and (2) nutrient enrichment of wetlands, which is one of the primary stressors damaging

wetlands in many parts of the country. This information is intended to serve as a starting point for States

and Tribes to eventually establish biological and nutrient water quality criteria specifically refined for

wetland waterbodies.

This purpose was to be accomplished by providing a series of “state of the science” modules concerning

wetland bioassessment as well as the nutrient enrichment of wetlands. The individual module format

was used instead of one large publication to facilitate the addition of other reports as wetland science

progresses and wetlands are further incorporated into water quality programs. Also, this modular

approach allows EPA to revise reports without having to reprint them all. A list of the inaugural set of

20 modules can be found at the end of this section.

This series of reports is the product of a collaborative effort between EPA’s Health and Ecological

Criteria Division of the Office of Science and Technology (OST) and the Wetlands Division of the

Office of Wetlands, Oceans and Watersheds (OWOW). The reports were initiated with the support

and oversight of Thomas J. Danielson (OWOW), Amanda K. Parker and Susan K. Jackson (OST),

and seen to completion by Douglas G. Hoskins (OWOW) and Ifeyinwa F. Davis (OST). EPA relied

heavily on the input, recommendations, and energy of several panels of experts, which unfortunately

have too many members to list individually:

n Biological Assessment of Wetlands Workgroup

n Wetlands Nutrient Criteria Workgroup

More information about biological and nutrient criteria is available at the following EPA website:

http://www.epa.gov/ost/standards

More information about wetland biological assessments is available at the following EPA website:

http://www.epa.gov/owow/wetlands/bawwg
vi

List of “Methods for Evaluating Wetland

Condition” Modules

1 ................. Introduction to Wetland Biological Assessment

2 ................. Introduction to Wetland Nutrient Assessment

3 ................. The State of Wetland Science

4 ................. Study Design for Monitoring Wetlands

5 ................. Administrative Framework for the Implementation of a

Wetland Bioassessment Program

6 ................. Developing Metrics and Indexes of Biological Integrity

7 ................. Wetlands Classification

8 ................. Volunteers and Wetland Biomonitoring

9 ................. Developing an Invertebrate Index of Biological

Integrity for Wetlands

10............... Using Vegetation to Assess Environmental Conditions

in Wetlands

11 ............... Using Algae to Assess Environmental Conditions in

Wetlands

12 ............... Using amphibians in Bioassessments of Wetlands

13 ............... Biological Assessment Methods for Birds

14 ............... Wetland Bioassessment Case Studies

15 ............... Bioassessment Methods for Fish

16 ............... Vegetation-Based Indicators of Wetland Nutrient

Enrichment

17 ............... Land-Use Characterization for Nutrient and Sediment

Risk Assessment

18............... Biogeochemical Indicators

19 ............... Nutrient Load Estimation

20 .............. Sustainable Nutrient Loading

Module # Module Title
1

Summary

T he condition of and potential threats to agiven wetland are often largely determined by

surrounding land use. Land use, especially the percent of the watershed that has been cleared of natural vegetation, can affect the amount of water, sediment, pesticide, and nutrients entering a wetland as

well as the composition of its plant and animal communities. A rapid and inexpensive assessment of

the land use around a wetland and its implications

for nutrient and sediment inputs can be made using

readily available maps and aerial photographs, including county soil maps, USGS quadrangle maps,

land-use maps, and aerial photographs. These are

used to delimit the watershed or drainage basin

around a wetland and to classify land use within this

watershed. These land use data are used to calculate an index of potential nutrient loading to the

wetland and a sediment risk index. Their intended

use is to identify quickly, easily, and cheaply those

wetlands that are at greatest risk from nutrient and/

or sediment inputs.

Purpose

T

he purpose of this module is to suggest

procedures for assessing the relative risk to a

given wetland of nutrient and sediment loads from

its watershed.

Introduction

A

ll wetlands are influenced by a number of

 flow systems that bring materials into and out

of them. The most important flow systems are usually atmospheric flows, surface water flows, and

subsurface water flows. Many natural flow systems have been altered by human activities, especially surface and subsurface water flows into wetlands. The amount of water (Luo et al. 1994, Euliss

and Mushet 1996), nutrient (Omernik 1977), sediment (Martin and Hartman 1987, Gleason and

Euliss 1998), and pesticide entering a wetland is

largely a function of land use in the watershed. Inputs into wetlands, especially sediment from surrounding areas, can affect the recruitment, growth,

and even survival of many plant and animal species

(Dieter 1991, Euliss and Mushet 1999, Jurik et al.

1994, Martin and Hartman 1987, Newcomb and

MacDonald 1991, Waters 1995).

The characterization of land use around a wetland is essential for evaluating wetlands for water

quality purposes. Data from maps and aerial photographs can be used to estimate the potential for

inputs of nutrients and sediments into wetlands.

Many factors determine the movement of nutrients

and sediments within most landscapes, including

vegetative cover (land use), soil type (erodibility),

slope length, slope angle, and frequency and intensity of rainfall. A variety of models and equations

exist for estimating the potential loads of nutrients

and sediments to a wetland from its surrounding

watershed. The data requirements of these models

generally make them unsuitable for preliminary

evaluations of the risk of nutrient or sediment inputs

to a wetland. Nevertheless, when reduced to its

simplest terms, the more a wetland’s watershed has

been altered by human activities the more likely it is

at risk. Consequently, a quick-and-dirty assessment of land use in the watershed should provide a

crude estimate of potential risks from nutrient and

sediment loadings to a wetland.

Methods

G

eographic information systems (GIS) are

primary tools in analysis of landscapes, and

their use has significantly changed how spatial data

and related nonspatial data are collected, stored,

and analyzed. State, regional, and local GIS facilities

will often have on file in digital form all the information

needed to characterize landscapes around a given

wetland. Appropriate State, county, and municipal

agencies should be consulted to see if their GIS

facilities can be utilized for this purpose. If a GIS
2

facility is not available, the work needed to

characterize the landscape in which a wetland is

found can be easily done by hand. The methods

outlined in this module presuppose that a GIS will

not be used. All of the proposed steps suggested

in this module, however, can be done utilizing existing

GIS software packages or simple modifications or

extensions of them. Although the proposed method

is simple, it will quickly become prohibitively timeconsuming if extended to a statewide assessment

of risks to wetlands. For large-scale projects, a

GIS should be used.

A very useful and practical manual that provides

detailed methodology for collecting the kinds of information needed to characterize lands use around

wetlands is Landscape Planning: Environmental Applications, 3rd Edition, by William M. Marsh.

Much of what follows is based on Marsh (1998).

Guidelines for selecting wetlands to sample are found

in Study Design for Wetland Monitoring.

There are many kinds of wetlands, including fringing

wetlands along lakeshores, rivers, and oceans, and

palustrine wetlands that cover their entire basin (see

module on Wetland Classification for details).

These wetlands vary greatly in size and shape and

in inputs and outputs of water. To simplify this module, the methods described are applied to the evaluation of the watershed of a palustrine wetland within

a well-defined watershed. This is an idealized situation. To apply this methodology to other wetland

types, suitable adjustment will need to be made. In

some cases, watershed boundaries will be difficult

or impossible to establish. In such cases, an arbitrary zone of influence around the wetland can be

substituted. This zone’s width needs to take into

account known surface and subsurface flow patterns into the wetland. For large wetland complexes,

e.g., riverine or estuarine wetlands, only portions of

the complex may be of interest. Again, suitable

adjustments will need to be made to determine the

zone of influence. In many cases, significant nutrient and sediment inputs may come from other sections of the complex.

Sources of Maps, Aerial

Photographs, and Other Data

The Internet has made finding and acquiring relevant maps and other data simpler and more efficient than ever before. Increasingly, digitized maps

can be downloaded directly. Below is a list of some

major sources of maps and aerial photographs.

These Websites often have links to other sites with

relevant information. Only the URLs for their home

pages are listed because these are less likely to

change. See also the module on Wetland Classification for additional sources of information.

Topographic maps are published by the U.S. Geological Survey (USGS) at a variety of scales. The

most useful topographic maps are the 7.5-minute

quadrangle maps. These maps contain information

on topographic relief, drainage systems, and some

land use features. The USGS Web site

(www.usgs.gov) provides a list of all topographic

maps and all other maps produced by the USGS.

It also describes several ways of ordering these

maps. Current and historic stream-flow data can

also be obtained from this site.

National Wetland Inventory (NWI) 7.5-minute

quad maps can be downloaded from their Website

(www.nwi.fws.gov) and are also available through

the USGS Earth Science Information Centers.

A list of all county soil maps in the United States

is available through the National Resources Conservation Service (NRCS) Website

(www.nrcs.usda.gov), as is a list of State NRCS

offices from which these county soil surveys can be

obtained. Information about regional soil erosion

patterns can also be obtained from this site.
3

Information about aerial photographs that are part

of the National Aerial Photography Program

(NAPP) can be obtained from the USGS Website

(nsdi.usgs.gov/products/aerial.html). Aerial

photographs taken by State agencies and local governments are also usually available. Recent aerial

photographs and land-use maps can usually be obtained from State agencies such as the State Geological Survey and map and aerial photography collections at major universities.

A National Land Cover Data Base (NLCD) based

on satellite imagery from the early 1990s is being

developed as part of the Federal Multi-Resolution

Landscape Characterization (MRLC) initiative. The

NLCD is a joint project of the USGS and the U.S.

Environmental Protection Agency (USEPA). Its

aim is to produce a consistent land cover data layer

for the conterminous U.S. based on 30-meter

Landsat thematic mapper (TM) data. Information

about available NLCD products can be obtained

from both USGS (edcwww.cr.usgs.gov/programs/

lccp/nationallandcover.html) and EPA Websites

(www.epa.gov/mrlc/Regions.html).

Flood hazard maps from the Federal Emergency

Management Agency (www.fema.gov) can often

be used to delineate the boundaries of floodplain

wetlands along rivers and streams.

Map Limitations

All maps have a minimum resolution level that is

determined by their scale. This establishes the smallest area or unit that can be mapped. For USGS

7.5-minute quadrangle maps, whose scale is

1:24,000 or 1” = 2,000 ft, the smallest units that

can be mapped are 10 or more acres. Because

many wetlands, especially palustrine wetlands, are

often much less than 10 acres and their watersheds

are equivalently small, USGS quadrangle maps may

not have the resolution needed to delineate their

watershed boundaries. In flat landscapes, there is

an additional limitation of USGS 7.5 quadrangle

maps, i.e., their vertical resolution. In short, for

small wetlands or wetlands in very flat landscapes,

maps with a better resolution than USGS quadrangle maps may be needed. Unfortunately, county

soils maps, which typically have a 1:20,000 scale,

have only a slightly better resolution than USGS

quadrangle maps. When no suitable maps are available, low-level aerial photographs may be the only

way to collect the data needed to characterize the

watershed in which the wetland is found.

Watershed or Drainage Basin

Delineation

Establishing the boundaries of a watershed around

a given wetland is done by finding drainage divides

on a topographic map. In landscapes with welldeveloped drainage systems, this process begins

with mapping the drainage network to establish the

order of its various branches. The results of this

exercise will largely be a function of the resolution

of the topographic map. One of the simplest ways

to identify drainage divides is to demarcate patterns

of overland flow with arrows drawn perpendicular

to contour lines. Where the arrows are divergent,

i.e., point in opposite directions, there is a drainage

divide. By inspection, first-order basins can be identified and aggregated into second-order watersheds

and so forth.

Identifying watersheds, as described above, may

not be feasible in some landscapes. In flat, poorly

drained landscapes, drainage divides are often hard

to determine using topographic maps. In such situations, soils maps may be more useful for identifying both wetland basins and intermittent, interconnecting drainage channels. In these landscapes,

groundwater inputs into wetlands are often very

important. When this is so, watershed delimitation

using surface flow patterns may greatly underestimate the effective size of the area around a wetland

whose runoff enters the wetland. In both urban

and agricultural areas, the effective watershed of a
4

wetland includes the area drained by storm sewers

or drainage networks, respectively, which discharge

into the wetland. The area covered by these storm

sewer or drainage networks generally is not congruent with the wetland’s surface runoff watershed.

Maps and other information about storm sewer and

drainage networks are often available from local

governments or from organized drainage districts.

These can be used to establish the effective watershed for wetlands in such altered landscapes, and

these effective watersheds should be used to estimate potential loading rates of nutrients and pollutants to the wetland.

Land Use in Watershed

Nutrient and sediment inputs into a wetland are

due to both point and nonpoint inputs. To a large

extent land use in the watershed will determine the

load of sediment and nutrients that a wetland will

receive. It will also determine how much of this

input is the result of point and nonpoint sources.

The more a watershed has been altered by human

activities, the greater the potential for the movement of sediments into a wetland. The first step in

characterizing the watershed is to classify each recognizable area in the watershed by its predominant

land use:

n Natural vegetation (>75% forest and/or grassland)

n Mostly natural vegetation (50 to 75% forest

and/or grassland)

n Agriculture (>75% cropland)

n Mostly agriculture (50 to 75% cropland)

n Mostly urban (>40% developed)

n Mixed (doesn’t fall into one of the previous

categories)

Because land use is changing constantly, up-todate information on current land use is needed. This

can most readily be obtained from recent aerial

photographs of the watershed. In some States, land

use maps may be available, but the classification

system used will undoubtedly be more sophisticated

than the simple system proposed above. Converting the classes on existing land-use maps to the landuse classes proposed above is usually very easy

and may not be necessary at all if local estimates of

annual nutrient loss for different classes are available. In agricultural areas, USDA crop compliance maps may be ready-made land-use maps. For

wetlands with large watersheds, the MRLC National Land Cover Data Base may also be a useful

source of digital land-use data.

The area of the watershed in each land use class

then needs to be determined. This can be done in a

variety of ways with paper maps using a dot grid or

planimeter. If a digital version of the land use map

is available, a GIS can be used to calculate the area

of each land use class in the watershed.

Dot grids are transparent overlays with a regular

pattern of dots. With a dot grid, the number of dots

that fall on each land use class in the watershed is

counted. Each dot represents a certain area. To

determine the area represented by a dot, the following formula is used:

Area/dot = [area on map/(1 linear unit)2]/

[number of dots/(1 linear unit)2 ]

For example, if 1 cm on the map is equivalent to

100 m, then 1 cm2 = 10,000 m2 or 1 ha.

If 1 cm2 on the dot grid contains 10 dots, then

Area/dot = (1 ha/ 1 cm2)/(10 dots/ 1 cm2)

= 0.1 ha/dot.

If 40 dots on the grid fell in areas that were covered with natural vegetation, the total area in natural vegetation is estimated to be 40 dots x 0.1 ha/

dot or 4 ha. To improve the precision of area estimates using dot counts, several random drops of

the dot grid can be made on the watershed map

and the dots counted for each land-use class. The

average dot count per class is then used to calculate the area of each land-use class.
5

A planimeter is a device that is used to convert

the boundary or perimeter length of a polygon to its

area. Prolonged use of a planimeter is tedious, and

it takes skill and practice to trace the boundaries of

complex polygons. For occasional use, dot grids

are simpler to use and probably more reliable. If

large number of wetlands are going to be evaluated, scanning land use maps to create digital versions and using a GIS to calculate total watershed

and land use class areas is strongly encouraged.

Nutrient Loading Index

A

 simple index of potential nutrient loadings can

be used to characterize the nutrient environment of a wetland. This index is the ratio of the

potential loss of nitrogen (N) or phosphorus (P)

from the watershed with its current land use divided

by the potential losses if the entire watershed were

covered with permanent natural vegetation. Operationally, potential nutrient loadings are calculated

by multiplying the area of each upland land use class

by the amount of N or P that is estimated to leave

this land use class (Table 1 or comparable local

data set) and then adding up the total annual estimated output of N or P for the watershed. (See

also the Vegetation-Based Indicators of Wetland

Nutrient Enrichment section of this manual for additional sources of information on nutrients in runoff.) This approach to estimating total annual loads

of nutrients from watersheds into lakes has been

used for many years in the limnological literature

(Reckow et al. 1980). The total estimated annual

loss of P or N is then divided by the estimated output of nutrient if the entire watershed were covered

in permanent natural vegetation (forests or grasslands).

Nutrient loss rate (NLR)

Land Use (LU) Nitrogen Phosphorus

Natural vegetation 0.44 0.0085

Mostly natural vegetation 0.45 0.018

Agricultural 0.98 0.031

Mostly agricultural 0.63 0.028

Mostly urban 0.79 0.030

Mixed 0.55 0.019

Source: Adapted from Omernik (1977) and Marsh (1998)

Table 1: Potential nutrient loss rates (kg/ha/yr) for

different upland land-use classes
6

ers or agricultural drainage networks will be highly

site specific and can best be obtained from local

sources. In areas where septic tank fields are within

100 m of a wetland, the number of septic tank fields

in the watershed needs to be estimated. The input

load to the wetland needs to be adjusted by determining the potential inputs of nitrogen (11 kg per

drainage field per year) and phosphorus (0.28 kg

per drainage field per year) as suggested by Marsh

(1998). Many other point sources of nutrients may

be present in a watershed, e.g., feedlots, golf

courses, sewage treatment plants, etc. In short,

estimates of point sources are simply added to those

from nonpoint sources. The Nutrient Loading Index is then calculated as above with estimated inputs of nutrients added to the numerator.

Sediment Risk Index

The simplest method for assessing potential sediment-related impacts to wetlands is to do an erosion risk assessment of the watershed. The major

factors that influence soil erosion rates are climate,

soil properties, topography, soil surface conditions,

and human activities. Within a given region, it is soil

surface conditions and human activity in close proximity to the wetland that will largely determine the

potential sediment load. Potential risk of sediment

inputs increases with the amount of land in the watershed that is classified as agricultural. It further

increases if this agricultural land has steeper and

longer slopes, i.e., is classified as highly erodible

land (HEL), and/or if it is adjacent to the wetland.

The amount of the watershed classified as HEL can

be obtained by contacting the nearest NRCS Office. How much of the land that is classified as

agricultural land is adjacent to the wetland can be

determined with a planimeter. In many watersheds,

recent clearing of natural vegetation due to road

building, construction, mining, lumbering, etc., can

also cause significant short-term erosion problems.

The potential impact of these activities is assessed

by estimating the total percentage of the watershed

recently cleared of natural vegetation.

(1) Nutrient Loading index = (Total

Watershed Loss)/(Natural Watershed

Loss)

where

(2) Total Watershed Loss = S(LUi x NLRXi)

LUi = area of watershed in upland land

use class i;

NLRXi = nutrient loss rate for upland land

use class i for nutrient X (Table 1);

and

(3) Natural Watershed Loss = TUWA x

NLRNVX

TUWA = total upland watershed area;

and

NLRNVX = nutrient loss rate of nutrient

X for the natural vegetation land use class

(Table 1).

The higher the index value, the larger the potential

nutrient loading into the wetland. This index provides a measure of the relative threat to the wetland

from nutrient inputs from the current upland landscape.

The index does not give a realistic estimate of the

actual annual loadings of nutrients to a wetland. It

only provides a crude way of ranking wetlands based

on the likelihood that they are receiving nutrient

loadings from the surrounding watershed. Other

sections of this manual discuss methods for estimating actual loadings of nutrients to wetlands.

If there are point sources of nutrients within the

watershed, the index needs to be adjusted appropriately. Annual inputs of nutrients from storm sew-
7

Sediment Risk Index = ( Percent of agricultural

land classified as HEL x percent of agricultural

land ) / 100 + Percent of wetland boundary

adjacent to agricultural land + Percent of

watershed disturbed by land-clearing activities.

As with the Nutrient Loading Index, the Sediment

Risk Index is a quick-and-dirty way to identify

wetlands that have a greater risk of having high sediment loadings. There are many sophisticated methods for estimating sediment losses from watersheds

(see Boardman and Favis-Mortlock 1998, Morgan 1986, Schmidt 2000). These should be used

to confirm that a given wetland is actually at risk

from sediment inputs.

Final Comments

In this module, overland flow and agricultural

drainage networks, if present, are assumed to be

the major sources of nutrients and sediments into a

wetland. For wetlands in watersheds where

stormwater sewers are a significant or predominant

source of nutrients and sediments, appropriate

changes in the methods for estimating nutrient and

sediment loadings will need to be made. Likewise,

better estimates of annual nutrient loss rates for areas with different land uses may be available locally. Because these rates are a function of local

soil types, fertilizer application rates, and precipitation patterns, these should be substituted for those

in Table 1 when available. In short, this module

presents some simple methods for gauging the overall

risk to a wetland of various human impacts on watershed land use. Users are encouraged to adapt

the proposed methodology to the realities of local

landscapes in order to improve its reliability.

Case Study

L

and-use data adapted from data supplied by

the NRCS Office in Spirit Lake, IA, for three

watersheds around the Iowa Great Lakes are presented in Table 2. Land use classes in the NRCS

data have been converted into the land-use classes

Land Use Watershed

of w1

Watershed

of w2

Watershed

of w3

Natural vegetation 87 69 349

Mostly natural vegetation 88 0 44

Agriculturea 132 43 2,797

Mostly urban 0 67 0

Mixed 12 26 57

Total Upland Watershed

Area (TUWA) 319 206 3,247

Wetlands and aquatic

systems

80 87 276

Total watershed area 399 293 3,523

Note: Data adapted from land-use data provided by the NRCS Office in Spirit Lake, IA.

a Agricultural land is all in row crops, mostly corn and soya beans.

Table 2: Land uses (ha) in the watersheds of wetlands W1, W2, and W3

in the Iowa Great Lakes Region of NW Iowa
8

presented in Table 1. W1 and W3’s watersheds

had land in the Conservation Reserve Program

(CRP). This CRP land was mostly planted to various kinds of perennial prairie grasses. In Table 2

CRP land has been classified as mostly natural vegetation. Based on percent of the upland area of

each watershed in agricultural land (41%, 21%, and

86% for the watersheds for wetlands W1, W2, and

W3, respectively), wetland W3 has the highest risk

of inputs of nutrients and sediments from the surrounding uplands, followed by W1 and W2.

The estimated annual loadings of nitrogen and

phosphorus to each wetland calculated using Equation 2 are presented in Table 3, as is the Nutrient

Loading Index for each wetland. Of the three wetlands, wetland W3 has the highest potential nutrient

loadings. This is largely because so much of its

watershed is in agricultural land. Wetlands in W1

and W2 have identical Nutrient Loading Indices.

For W1, however, the largest potential source of

nitrogen and phosphorus is agricultural land while

for W2 it is runoff from urban areas.

Data used to calculate the Sediment Risk Index

are given in Table 4, as is the Index. Wetland W2

has the highest Sediment Risk Index (56%), although

it is nearly identical to that for W3 (53%). Wetland

W2 is primarily at risk because so much of it (42%

of its shoreline) is adjacent to agricultural land and

because there is a lot of recently cleared land in its

watershed (10%). On the other hand, wetland W3

is at risk primarily because so much of the watershed is agricultural land (86%) and about 22% of

this land is classified as HEL

Collectively, the four indicators of the potential risk

of wetlands to nutrient and sediment inputs (percent of agricultural land in the upland portions of

the watershed, Nutrient Loading Indices for N and

P, and Sediment Risk Index) suggest that wetland

W3 overall is at greatest risk, followed by W2 and

W1.

Land Use nitrogen phosphorus

w1 w2 w3 w1 w2 w3

Natural vegetation 38 30 154 0.74 0.59 2.97

Mostly natural vegetation 40 0 20 1.6 0 0.8

Agricultural 129 42 2,741 4.1 1.3 86.7

Mostly urban 0 53 0 0.0 2.0 0.0

Mixed 7 14 31 0.2 0.5 1.1

Wetland and aquatic systems 0 0 0 0.0 0.0 0.0

Total watershed loss 214 140 2,946 6.6 4.4 91.6

Natural watershed loss 140 91 1,429 2.7 1.8 27.6

Nutrient loading index 1.5 1.5 2.1 2.4 2.4 3.3

Table 3: Estimated current annual nitrogen and phosphorus

loadings (Total Watershed Loss), presettlement loadings

(Natural Watershed Loss), and Nutrient Loading Index for

three wetlands (W1, W2, and W3) in the Iowa Great lakes region
9

Landscape

Characteristics W1 W2 W3

Percent in agricultural land 41% 21% 86%

Percent of agricultural land that

is HEL 9% 18% 22%

Percent of wetland boundary

adjacent to agricultural land 25% 42% 29%

Percent of watershed disturbed

by land clearing 0% 10% 5%

Sediment Risk Index 29% 56% 53%

Table 4: Percent of the watersheds for wetlands W1, W2, and W3

classified as agricultural land, percent of agricultural land

classified as highly erodible (HEL), percent of wetland boundary

adjacent to agricultural land, and percent of watershed disturbed by

recent land clearing for three watersheds in the Iowa Great Lakes

region as well as the Sediment Risk Index for each wetland
10

References

Morgan RPC. 1986. Soil erosion and conservation.

United Kingdom: Longman, Harlow. 298pp.

Newcombe CP, MacDonald DD. 1991. Effects of

suspended sediments on aquatic ecosystems. North

Am J Fisheries Manage 11:72-82.

Omernik JM. 1977. Nonpoint source—stream nutrient

relationships: a nationwide study. U.S. Environmental

Protection Agency, Corvallis, OR. 150 pp.

Peterson SA, Carpenter L, Guntenspergen G, Cowardin

LM. 1996. Pilot test of wetland condition indicators in

the prairie pothole region of the United States. U.S.

Environmental Protection Agency, Washington, DC.

EPA/620/R-97-002.

Reckow KH, Beaulac MN, Simpson JT. 1980. Modeling

phosphorus loading and lake response under uncertainty: a manual and compilation of export coefficients. U.S. Environmental Protection Agency,

Washington, DC. EPA 440/5-80-011.

Schmidt J (ed). 2000. Soil erosion: application of

physically based models New York: Springer. 318 pp.

Waters TF. 1995. Sediment in streams: sources,

biological effects, and control. Am Fisheries Soc

Monogr 7.

Boardman J, Favis-Mortlock D (eds). 1998. Modeling

soil erosion by water. New York: Springer. 531 pp.

Dieter CD. 1991. Water turbidity in tilled and untilled

prairie wetlands. J Freshwater Ecol 6:185-189.

Euliss NH, Mushet DM. 1996. Water-level fluctuation

in wetlands as a function of landscape condition in the

prairie pothole region. Wetlands 16:587-593.

Euliss NH, Mushet DM. 1999. Influence of agriculture

in aquatic invertebrate communities of temporary

wetlands in the prairie pothole region of North Dakota,

USA. Wetlands 19:578-583.

Gleason RA, Euliss NH. 1998. Sedimentation of prairie

wetlands. Great Plains Res 8:97-112.

Jurik TW, Shih-Chin Wang, van der Valk AG. 1994.

Effects of sediment load on seedling emergence from

wetland seed banks. Wetlands 14:159-165.

Luo H-R, Smith LM, Allen BL, Haukos DA. 1997.

Effects of sedimentation on playa wetland volume.

Ecol Appl 7:247-252.

Martin DB, Hartman WA. 1987. The effect of cultivation on sediment and deposition in prairie wetlands.

Water Air Soil Pollut 34:45-53.

Marsh WM. 1998. Landscape planning: environmental

applications. 3rd ed. New York: Wiley. 434 pp.